Double focussing mass spectrometer including a wedge-shaped magnetic sector field



May 20, 1969 H. J. LIEBL 3,445,650

DOUBLE FOCUSSING MASS SPECTRQMETER INCLUDING A WEDGE-SHAPED MAGNETIC SECTOR FIELD Filed Oct. 11, 1965 INVENTOR.

HELMUT J. LIEBL FIG. 6

ATTORNEY United States Patent 6 U.S. Cl. 250-419 6 Claims ABSTRACT OF THE DISCLOSURE A mass spectrometer including a wedge-shaped, 90, magnetic sector lens in tandem with a 45, spherical, electrical condenser lens.

This invention relates to a novel mass spectrometer of the double focussing type, and, more particularly, to a novel mass spectrometer including magnetic and electric focussing elements that complement each other to produce a stigmatic image of a particulate source.

Mass spectrometers are widely used for chemical analysis and isotope separation. They frequently consist of two focussing elements, or lenses, an electric lens and a mag netic lens, in tandem. The electric lens disperses particles in accordance with their respective energies, and concentrates particles having energies within a given range, or band into a predetermined region. The magnetic lens disperses the particles in accordance with their momenta, which under the usual conditions correspond to their masses.

In most cases the sequence of the two lenses is considered immaterial, and either lens may be first traversed by the charged particles under analysis.

A major problem with instruments of this type has to do with maximizing their resolving power while still maintaining satisfactory sensitivity and low background noise levels. In the usual case, where good resolving power is desired, it has heretofore been necessary to use an instrument having a relatively small aperture and therefore capable of accepting only a relatively small proportion of the charged particles available initially for analysis, with the result that the sensitivity of such instruments is relatively low.

Accordingly, the principal object of the present invention is to improve the art of mass spectrometry by providing a novel mass spectrometer having good resolution, and stigmatic imaging characteristics, together with a relatively large acceptance angle, large aperture and relatively high transmission factor, and therefore capable of a greater sensitivity than heretofore achievable with comparable resolving power.

The invention will now be described in greater detail in connection with the accompanying drawing, wherein:

FIGURE 1 is a schematic plan view of the magnetic sector field lens of the mass spectrometer of the invention;

FIGURE 2 is a cross-sectional view, taken along the line 22 of FIGURE 1;

FIGURE 3 is a view generally similar to the view of FIGURE 1, but showing the effect of momentum dispersion;

FIGURE 4 is a partly schematic, thin horizontal sectional view of a spectrometer according to a presently preferred embodiment of the invention;

FIGURE 5 is a rear elevational view of the spectrometer shown in FIGURE 4; and

FIGURE 6 is an end elevational view of the spectrometer shown in FIGURES 4 and 5.

Briefly, the invention contemplates the combination of a wedge-shaped magnetic field having an arcuate main Patented May 20, 1969 ICC axis extending through about in combination with a spherically curved electric condenser lens, the main axis of which extends through about 45. It has been found that this combination is particularly advantageous, not only in its performance characteristics, but also in that it is relatively simple and inexpensive to manufacture.

Referring now to the drawing, the spectrometer of the present invention includes a magnetic sector field of wedge shape, of the general type described by I. S. OConnell in the Review of Scientific Instruments, volume 32 (1961),

page 1314. The present field arrangement is novel, however, in that its dimensions are so chosen that it has a stigmatic focal point and no second order angular image aberration.

The field is defined by the faces 10 and 12, respectively, of a pair of pole pieces 14 and 16, which may be energized by any desired means (not shown) preferably by an electromagnet, so that its strength may be easily varied. For purposes of the following discussion, the pole faces 10 and 12 may be thought of as rectangular in shape, and as being inclined relative to each other with their upper and lower edges parallel and lying in common respective planes normal to the plane of symmetry of the wedge. The principal coordinate axis of the system is defined as the line 20 of intersection of the planes in which the pole faces 10 and 12, respectively, lie. In a practical instrument, the angle between the pole faces 10 and 12 would be about 10 to 20 depending upon the amount of power available for energizing the field. As the angle is made larger, greater power is required to achieve a given mean field strength. As is known, in a wedge-shaped field the field lines, indicated at 22, are in the form of circularly curved arcs centered on thecoordinate axis 20. Because of this configuration, the field focusses ions passing through it not only in the radial direction, i.e., in the plane of FIG- URE 1, but also in the axial direction, i.e., in the plane of FIGURE 2. The field strength decreases proportionately to the distance from the coordinate axis 20, and may be expressed:

where F is a proportionality constant, and r is the distance from the axis 20. Charged particles describe t-rochoidal paths in such a field, not circular paths.

An important feature of the invention pertains to the relative dimensions of the field, which may be most simply described with reference to FIGS. 1 and 2, and with reference to ions of a selected momentum that enter the field through its narrow edge 29 along paths within a solid angle, 0, centered about a selected normal 23 to the lower edge 29 and about equal in magnitude to the wedge angle of the field. The ions are assumed to come from a point source 24 located on the selected normal 23.

It has now been found that all such ions will leave the field after a mean deflection of 90 travelling along paths parallel to the coordinate axis 20, provided that the source 24 is placed on the axis 20 at a critical distance B from the boundary plane of the field through which the ions leave the field. In the example shown in FIGURE 1, this is the plane defined by the right hand edges 26 of the pole pieces. The critical distance is about 1.347 times the distance A between the axis 20 and the field, and the median plane 32 of the emergent ions is spaced from the coordinate axis 2.100 times the distance between the axis 20 and the field. The paths of the ions emerging from the field are then parallel both in the first and in the second approximation. As a corollary, when the field is passed in the opposite direction, and ions travelling along paths parallel to the coordinate axis 20 and close to the median plane 32 enter the field from the right, as viewed in FIG. 1, they will be stigmatically focussed at the source 24 without second order aberration.

In practice, the pole faces 10 and 12 need not be rectangular, but are preferably curved to conform generally to the curvature of the ion paths in order to minimize the mass of the pole pieces and the power required for producing the magnetic field. They extend radially slightly beyond the limits of the ion paths in order to avoid the effects of fringe fields.

An important advantage of this magnetic field arrangement lies in the substantially complete utilization of the space between the pole faces 10 and 12. The ion paths fill the entire wedge and do not cross each other.

In the mass spectrometer of the invention, the magnetic sector lens acts as a momentum dispersing element, and is combined in tandem with an electric lens, which acts as an energy dispersing element. By selecting a spherical condenser for the electric lens, an optimum combination is achieved resulting in a mass spectrometer that is double focussing (in the sense meaning simultaneous angular and energy focussing), stigmatic imaging (meaning that the first order image point in the radial plane coincides with the first order image point in the axial plane), has large solid angle. collection efiiciency (relatively large optical aperture), and is also easy to manufacture.

In general, in designing a mass spectrometer of the double focussing type, it is necessary to match the momentum dispersion of the magnetic field with that of the electric field. Although the electric field is an energy dispersing device, a momentum dispersion factor may be calculated for it which is applicable for ions of any selected mass.

The momentum dispersion of the magnetic field of the present invention is illustrated in FIGURE 3. It can be shown that ions with the momentum p=p (l+u), where u 1 are defected less in the magnetic field than ions with the momentum p by the angle:1.39u radian. The calculation shows that to match this dispersion factor, the sector angle of the spherical condenser should be 44. For convenience in manufacture, however, the sector angle of the spherical condenser may be 45, and exact energy focussing obtained by adjusting the mean potential of the spherical condenser field.

FIGURES 4, 5 and 6 illustrate the basic geometry of the mass spectrometer of the invention including the 90 magnetic sector field 40, and the 45 electric spherical condenser 42. The sequence of fields may be passed by the ion beam in either direction. For convenience in the present description, it will be assumed that the ions are moving from right to left, as viewed in FIGURES 4 and 5, from a source 41 defined by an apertured entrance diaphragm 43, passing through the electric lens 42 first and then through the magnetic lens 40 to an image point 44 defined by an apertured exit diaphragm 46.

A narrow bundle of ions from the source 41 is selected by the apertured collimating diaphragm 48 for admission into the spherical condenser 42. The spherical condenser 42 disperses the ions in accordance with their energies. Ions having energies within a selected limited range pass through the selector diaphragm 50, which is placed between the electric and magnetic fields 42 and 40, respectively. The magnetic sector lens 40 then disperses the ions in accordance with their momenta, or masses, and those ions having the selected momentum are stigmatically focussed at the image point 44.

If the fields are passed in the opposite direciton, momentum dispersion is carried out first, and the selector diaphragm 50 acts as a momentum selector rather than an energy selector.

As hereinabove stated, in order to minimize power requirements for the magnetic field and the mass of the magnetic structure, those portions of the pole pieces 14 and 16 Where no field is needed are omitted, as seen in FIG. 4. This does not change the distribution of the magnetic field between the pole pieces 14 and 16, which remains as hereinabove described in connection with FIG- URES 1 and 2.

Because the spectrometer of the invention produces a stigmatic image subject only to relatively small image aberrations, it may be designed with a larger angular aperture than previous mass spectrometers of the same resolving power. This provides improved sensitivity, because a greater proportion may be used of the ions coming from the source than in instruments heretofore available.

Manufacture of the spectrometer is also relatively simple and inexpensive. No complicated surfaces need be produced. The surfaces for forming the magnetic field are planes, and those for forming the electric field are spherical. It is not only easy to make elements of these simple shapes, it is also much easier to assemble them and adjust their positions relative to each other than to assemble and adjust elements of more complicated or irregular shapes such as have heretofore been proposed.

A further advantage of the spectrometer of the invention is that both the source and the image are located well away from the dispersing fields, thus avoiding shielding problems such as might otherwise be encountered when using a field sensitive device at the image or at the source, for example, an electron multiplier at the image point.

The dimensions and approximate typical operating parameters of an actual embodiment of the invention are as follows. Electric potentials are given relative to ground.

Spherical condenser 42:

Mean radius 10 inches. Spacing between plates 1 inch. Potential of inner plate 250 volts.

Potential of outer plate (for positive ions of 2.5 kv.

energy) +250 VOltS. Distance from the source 24 to the entrance aperture 48 14 inches.

Magnetic sector lens 40:

Field strength at the median plane 32 Up to 9,000 gauss. Field strength at the narrow edge 27 Up to 19,000 gauss.

The distance A from the narrow edge 27 to the coordinate axis 5 inches. Mass resolving power Adjustable from to 10,000. Mass range Up to mass 1,000 at 1.5 kv. ion energy.

The entrance aperture 48, the selector aperture 50, and the exit aperture 46 are preferably all adjustable in size to permit adjustment to the optimum operating condiitons for ions of various diiferent energies and momenta, and of various different concentrations at the source 41. In general, the apertures will be adjusted together to maximize the mass resolving power to the limit set by the sensitivity desired.

The mass spectrometer of the invention is included in a microanalyzer described and claimed in my copending patent application filed concurrently herewith, Ser. No. 494,388, entitled Ion Microprobe, wherein it is shown in combination with an embodiment of an invention described and claimed in another copending application field concurrently herewith, Ser. No. 494,490, entitled, Lens System for Particulate Radiation.

I claim:

1. A magnetic sector lens for deflecting moving charged particles comprising means for producing a wedge-shaped magnetic field characterized by curved field lines that pass normally through the plane of symmetry of the wedge, said field having planar entrance and exit boundaries, one of said boundaries being parallel to the planes of curvature of the field lines, the other one of said boundaries lying along the narrow edge of the wedge and being normal to said one boundary and to the plane f symmetry, means for directing ions into a field produced by said producing means through one of said boundaries generally normally thereto, and means for utilizing ions emerging from the field through the other one of said boundaries generally normally thereto, said field being arranged to deflect ions having a selected momentum from said directing means along trochoidal paths between said boundaries, said paths being grouped about a median path that passes normally through both of said boundaries.

2. A magnetic sector lens for deflecting electrically charged particles comprising means for producing a wedge-shaped magnetic field characterized by curved field lines that pass normally through the plane of symmetry of the wedge, the line of intersection of the planes of the major faces of the wedge being defined as the coordinate axis of the lens, said field having a first boundary plane normal to said coordinate axis, and a second boundary plane lying along the narrow edge of the wedge and normal both to said first boundary plane and to the plane of symmetry, said field being substantially uniform throughout a predetermined area of said second boundary plane centered at a distance from said first boundary plane equal to about 1.347 times the distance between said second boundary plane and said coordinate axis, the strength of said field varying substantially uniformly ininverse proportion to distance from said coordinate axis throughout a predetermined area of said first boundary plane cencentered at a distance from said second boundary plane equal to about 1.10 times the direction between said second boundary plane and said coordinate axis, means for directing ions into a fiield produced by said producing means through one of said predetermined areas of said boundary planes in directions generally normal to the boundary plane, and means for utilizing ions emerging from the field through the other one of said predetermined areas.

3. A magnetic sector lens for deflecting electrically charged particles comprising a pair of pole pieces each having a planar face, means mounting said pole pieces with said planar faces confronting each other and defining a wedge-shaped space symmetrical about a plane midway between them, said planar faces having straight edges in each of two planes that are normal to each other and normal to said plane of symmetry, one of said planes of said straight edges lying along the narrow edge of the wedge-shaped space, means defining a median particle path in said plane of symmetry extending normally through said one plane of said straight edges and spaced from the other plane of said straight edges a distance equal to about 1.347 times the distance between said one plane and the line of intersecton of the planes of said planar faces, said pole faces extending in said other plane of said straight edges on both sides of a point space from said one plane 1.1 times the distance from said one plane to said line of intersection, whereby when said pole pieces are energized to establish a magnetic field between them, charged particles directed into the field with a predetermined momentum along said median particle path and in directions close to it sulfer a mean deflection of 90 and emerge along paths substantially parallel to said line of intersection, means for directing ions toward the region between said pole pieces along said median particle path and in directions close to it, and means for utilizing ions that emerge from between said pole pieces along said median particle path. I

4. A magnetic sector lens for deflecting charged particles comprising a pair of pole pieces having a planar face, means mounting said pole pieces with said planar faces confronting each other and defining a wedge-shaped space symmetrical about a plane midway between them, said planar faces having straight edges in each of two planes that are normal to each other and normal to said plane of symmetry, one of said planes of said straight edges lying along the narrow edge of the wedge-shaped space, means defining an aperture confronting the trapezoidal face of said wedge-shaped space bounded by two of said straight edges, said aperture being centered a distance from said one plane of said straight edges equal to about 1.10 times the distance from said one plane to the line of intersection of the planes of said planar faces, said straight edges in said one plane extending on both sides of a point spaced from the plane of said trapezoidal face a distance equal to about 1.347 times the distance from said one plane to said line of intersection, whereby when said pole pieces are energized to produce a magnetic field between them and ions are injected through said aperture into the field with a selected momentum and along paths parallel to said line of intersection, they suffer a mean deflection of and are brought to a stigmatic focus at a point on said line of intersection directly opposite from said spaced point, means for directing ions into the space between said pole pieces through said aperture or through an area portion of said one plane centered about said point, and means for utilizing ions that emerge from between said pole pieces through the other of said aperture and said area portion.

5. A mass spectrometer comprising means for producing a wedge-shaped magnetic field characterized by curved field lines that pass normally through the plane of symmetry of the wedge, said field having a planar entrance and exit boundaries, one of said boundaries being parallel to the planes of curvature of the field lines, the other one of said boundaries lying along the narrow edge of the wedge and being normal to said one boundary and to the plane of symmetry, said field being arranged to deflect ions having a selected momentum along trochoidal paths between said boundaries, said paths being grouped about a median path that passes normally through both of said boundaries, and a spherically curved, toroidal condenser having a planar boundary confronting and aligned with the boundary of said magnetic field that is parallel to the planes of the magnetic field lines, said condenser having a sector angle of about 45, means for directing ions into one of said magnetic lens and said condenser for passage through and deflection by both of them in sequence and means for detecting ions after passage through said magnetic lens and said condenser.

6. A mass spectrometer comprising means for producing a wedge-shaped magnetic field characterized by curved field lines that pass normally through the plane of symmetry of the wedge, said field having planar entrance and exit boundaries, one or said boundaries being parallel to the planes of curvature of the field lines, the other one of said boundaries lying along the narrow edge of the wedge and being normal to said one boundary and to the plane of symmetry, said field being arranged to deflect ions having a selected momentum along trochoidal paths between said boundaries, said paths being grouped about a median path that passes normally through both of said boundaries, and a spherically curved, toroidal condenser having a planar boundary confronting and parallel to the boundary of said magnetic field that is parallel to the planes of the magnetic field lines, said condenser having a sector angle of 45, and means defining an aperture between said magnetic field means and said condenser for restricting the passage of ions between said field and said condenser, means for directing ions into one of said magnetic lens and said condenser for passage through and deflection by both of them in sequence and means for detecting ions after passage through said magnetic lens and said condenser.

References Cited UNITED STATES PATENTS 2,947,868 8/1960 Herzog 250-419 3,061,720 10/1962 EWal-d 25041.9

OTHER REFERENCES Simple Broad-Range Magnetic Spectrometer, by J. S. OConnell, from The Review of Scientific Instruments, vol. 32, No. 12, December 1961, pages 1314-1316.

WILLIAM F. LINDQUIST, Primary Examiner.

US. Cl. X.R. 250-495 

